patch 8.0.1638: popup test fails depending on environment variable

Problem:    Popup test fails depending on environment variable.
Solution:   Reset $COLORFGBG when running Vim in a terminal. (closes #2693)

src/testdir/screendump.vim

@@ -30,6 +30,13 @@ func RunVimInTerminal(arguments, options)
  " Remove it here.
  call delete(".swp")

  if exists('$COLORFGBG')
    " Clear $COLORFGBG to avoid 'background' being set to "dark", which will
    " only be corrected if the response to t_RB is received, which may be too
    " late.
    let $COLORFGBG = ''
  endif

  " Make a horizontal and vertical split, so that we can get exactly the right
  " size terminal window.  Works only when we currently have one window.
  call assert_equal(1, winnr('$'))

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    1638,
/**/
    1637,
/**/